Solutions for the deposition of protective layers on zinc surfaces and process therefor



United States Patent 3,539,403 SOLUTIONS FOR THE DEPOSITION 0F PROTEC- TIV E LAYERS 0N ZINC SURFACES AND PROC- ESS THEREFOR Christian Ries, Cologne-Kalk, Germany, assignor to Gerhard Collardin GmbH, Cologne-Ehrenfeld, Germany No Drawing. Filed Nov. 20, 1967, Ser. No. 684,517 Claims priority, application Germany, Dec. 7, 1966, C 40,903 Int. Cl. C231? 7/00 US. Cl. 1486.14 13 Claims ABSTRACT OF THE DISCLOSURE Solutions for the deposition of protective layers on zinc surfaces which consist of aqueous solutions of complex fluorides of iron, titanium, zirconium or silicon and small amounts of nitrate ions or other oxidizers. The pH of these solutions is to be kept between 0.8 and 3.5. The solutions deposit a layer very rapidly, i.e., within 3 to 120 seconds, at temperatures of 5 to 95 C., and preferably at room temperature. Application can be by immersion, spray or roller coating. The protective layers produced impart to the surfaces good corrosion resistance and good adhesion of organic coatings subsequently applied. A passivation post-treatment may be applied in the form of dilute aqueous chromic and/ or phosphoric acid or acid salts thereof.

The invention relates to the deposition of protective layers on zinc or zinc-clad (galvanized) surfaces by means of acid solutions, and to a process therefor.

The application of protective layers on such surfaces by means of acid solutions is known for the purpose of improving the corrosion resistance and the adhesion of organic coatings subsequently applied, such as paints, lacquers or plastic, i.e., synthetic, coatings. Hitherto, phosphatizing or chromatizing solutions have been used. These procedures are not fully satisfactory because the phosphate layers, while providing good protection against corrosion, inhibit the adhesion of the organic coatings which makes itself felt particularly when thereafter a shaping or deformation step is applied. This effect is not as pronounced with chromatized zinc; however, chromatization has the drawback that the large quantities of effluents require decontamination so that relatively high expenses are incurred. Moreover, the adhesion of organic coatings still leaves much room for improvement after chromatization. This is especially true of hotor pot-galvanized steel surfaces.

The use of nitric acid solutions for the brightening of cadmium surfaces also is known. These solutions are to prevent tarnishing of the surfaces. These aqueous solutions contain, beside nitric acid, chromium-III-ions and those of fluorides. However, they furnish no protective layers but merely shiny surfaces. On zinc, neither corrosion resistance nor adhesion of organic coatings is provided with these solutions.

It is an object of the invention to present a simple process for the deposition of protective layers on zinc or zinc-clad surfaces which meets all requirements of the art, particularly with respect to corrosion resistance and adhesion of organic coatings, subsequently applied.

Another object of the invention is to provide suitable aqueous solutions to carry out that process.

3,539,403 Patented Nov. 10, 1970 ICC These objects are attained by treating the metal surface swith an aqueous solution which is free from chromic-, phosphoricand oxalic acids, and contains complex fluorides of iron, titanium, zirconium and/or silicon and at least one oxidizer. The solution has a pH of 0.8 and 3.5 and is applied between 5 and C., e.g., at ambient or room temperatures, until a protective layer of the desired thickness has formed. The treatment time, depending on the predetermined thickness of the layer, ranges from 3 to seconds.

Especially suited are aqueous solutions of the complex fluoride of iron. These produce particularly hard and durable layers which impart good corrosion resistance and also excellent adhesion of organic coatings, i.e., paints, lacquers and plastic (synthetic) coatings even when deformation subsequently is practiced.

The metal ions are incorporated in the aqueous treating solutions in the form of their water-soluble salts whereby, in the case of titanium, zirconium and silicon, the fluoride complexes are most applicable. It also is feasible to form the fluoride complexes in situ within the baths by incorporating therein soluble metal salts and the corresponding amounts of fluoride ions. For instance, titanyl sulfate or zirconyl chloride may be employed. Iron most opportunely is added as sulfate, nitrate or chloride. The treatment solutions may contain mixtures of metal ions or mixtures of metal salts with different anions. When the fluoride ions are not introduced as complex fluorides, they may be entered as hydrofluoric acid or its neutral or acid Water-soluble salts. A slight excess of fluoride ions over the stoichiometrical amount required for complex formation frequently is desirable.

Preferred oxidizers are those known as accelerators in phosphatizing solutions, such as water-soluble nitrates, nitrites, chlorates, bromates, hydrogen peroxide and its adducts, aliphatic and aromatic nitroand nitrosocompounds, e.g., nitroguanidine, picric acid, nitrophenols, monoand dinitrobenzenesulfonic acids. Mixtures of several oxidizing agents may be used. The quantity employed depends upon the concentration of the bath solution and the strength of the ozidizer. Generally, as effective amounts, quantities are employed which correspond in their oxidizing effect to 0.3 to 50 g./l. (grams per liter) nitrate ions.

Those aqueous solutions have been found particularly suited which contain 0.1 to 10 g./l. iron-III-ions, 0.2 to 40 g./l. fluoride ions and oxidizers corresponding to 0.3 to 50 g./1. nitrate ions. These are the efliective amounts of the bath ingredients and are correspondingly valid for the other compounds, i.e., titanium, zirconium and/ or silicon, or mixtures thereof.

In another preferred embodiment, nitrate ions are used as oxidizer. This permits operation at particularly low temperatures and short times thus renders the process especially sim le. Thus, in the simplest embodiment of the invention an aqueous solution of iron-III-nitrate and hydrofluoric acid can be used. The solutions are to be practically free of chromic acid or chromium-VI-compounds, oxalic acid or oxalates, and phosphoric acid or phosphates. These substances or their anions, respectively, inhibit the formation of the protective layers according to the invention because they are layer-forming anions themselves. Even relatively small quantities of, e.g., phosphates, decrease the adhesion of organic coatings. Chromic acid and chromium-VI-compounds, moreover, are

poisonous, and the effluents must be decontaminated.

The pH of the solutions should be 0.8 to 3.5. Below 0.8, uniform formation of the protective layer is not assured. The upper limit is set by lack of solubility or stability, respectively, of the complex fluorides so that the pH should not exceed 3.5 albeit a layer formation still takes place at this value.

When the pH is kept between 1.2 and 2.5, protective layers having the best properties are obtained. Low pH values accelerate the formation of these layers, and higher pH leads to somewhat softer layers. It is to be noted that good effects still are obtained with a pH of 0.8. The adjustment of the pH can be carried out with aqueous alkali or acids.

The protective layers form within very short periods of time. With suitable pH, i.e., low pH, a uniform and sufficiently heavy layer can be deposited in three seconds. When the pH is higher or the solutions very dilute, or when heavy protective layers are to be produced, the treatment time is longer, Preferred effective times are 3 to 120 seconds.

The bath temperatures are 95 C., and preferably substantially 50 C., i.e., ambient or room temperatures. This constitutes an additional advantage of the process because the baths need not be heated nor necessarily maintained at constant temperature.

The solutions can be applied to the surfaces to be treated by immersion, coating or spraying. Coating rolls can successfully be employed. The metal surfaces are cleaned and degreased, prior to the treatment, using the customary organic solvents or alkaline, neutral or acid cleansers. The pretreatment is governed by the degree of soiling of the materials.

In many instances, a passivation of the layers deposed is desirable. This can be accomplished by using dilute aqueous chromic and/or phosphoric acid in concentrations of 0.01 to 5 g./l. after the treatment just described.

In a preferred embodiment, the protective layers are treated for passivation with dilute aqueous chromic acid which contains chromium-III-ions. The effective concentrations generally are 0.2-2 g./1. CrO and 005-1 g./l. Cr O The surfaces subsequently are dried. They opportunely are rinsed with water prior to passivation, however, this is not a strict requirement particularly when squeeze rolls are used.

The treatment solutions may be produced by dissolving the required compounds or by dilution of corresponding concentrates. They have a long working life and can be replenished or renewed by addition of starting components to the constant point. Points of the solution are understood to be the quantities of 21/10 aqueous NaOH in milliliters (1111.) which are consumed by 10 ml. bath solution to effect a color change of bromophenol blue (free acid) or phenolphthalein (total acid). When the baths are renewed with the same liquid or solid concentrates, an adjustment of the pH may eventually become necessary.

For this reason, the solutions preferably are renewed with solutions or concentrates, respectively, which contain complex fluorides of iron, titanium, zirconium and/ or silicon and at least one oxidizer and which have a ratio of free acid to total acid of l:1.051.75.

The solutions, by the process according to the inven tion, deposit on zinc and zinc-clad surfaces uniform, thin and well-deformable protective layers of gray to yellowish color which impart good corrosion resistance and excellent adhesion of organic coatings. The process is easy to carry out, and the solutions have long working life. Decontamination of the efiluents, as required e.g., in chromate-containing solutions, is not needed.

The process according to the invention has been found particularly useful in continuous devices for sheet and coiled material and especially for the deposition of protective layers or hotor pot-galvanized metal.

The invention now will be more fully explained by the following examples. However, it should be understood that these are given merely by way of illustration, and not of limitation, and that it is intended to cover all modifications and embodiments of the invention which do not constitute a departure from the spirit and the scope thereof, as hereinafter claimed.

In the examples, temperatures are in degrees centigrade. Percentages are by weight. The solutions are aqueous.

EXAMPLE 1 A 5% solution of the following composition was pre pared for treatment of zinc-clad surfaces:

12.5% iron-III-nitrate-9 H O 5.31% HF 82.19% water The pH was adjusted to 1.6 with NaQH.

Hot-galvanized steel sheets, which had been cleaned with an alkali solution followed by a water rinse, were immersed in this solution for 15 seconds at 18. They were then rinsed with water and passivated for 10 seconds with a solution of 0.2 g./l. chromium trioxide at 60 and dried with a stream of hot air.

The galvanized sheets thus treated had a uniform, hard, light gray to yellowish protective layer which was an excellent base for organic coatings and imparted good corrosion resistance.

A PVC (polyvinylchloride) organosol was applied to these sheets, using a tie coat. For purposes of comparison, galvanized metal sheets were coated in the same manner which had been chromatized (yellow chromate) in the conventional manner. The respective coated sheets were given a star cut and subjected to an Ericssen ductility test of 6.5 mm. Thereafter, it was not possible to peel the PVC coating from the sheets treated according to the invention despite the deformation which they had experienced. In contrast thereto, the PVC coating could be peeled off the conventionally treated sheets.

EXAMPLE 2 A solution as described in Example 1, was used for the treatment of hot dip galvanized iron sheets. The degreased sheets were immersed for 10 to 20 seconds at room temperature and had a good and uniform protective layer.

In the titration of 10 ml. solution with 11/10 NaOH using phenolphthalein, 12 ml. thereof were consumed (total acid).

The solution was renewed to constant point with a concentrate containing 4% Fe(No -9 H 0 3% HF (40%) 3% HNO (60%).

The layer formation remained faultness after frequent renewal of the solution and high bath load.

EXAMPLE 3 Hot dip galvanized steel sheets having a typical frost pattern were sprayed after alkaline degreasing for 15 seconds at 60 at a spray pressure of 2 atmospheres followed by a cold water rinse. The passivation was carried out with a solution containing 0.5 g./l. iron-III-ions, 10 g./l. fluoride ions and 0.6 g./l. nitrate ions. The spray solution had a pH of 1.4 (adjusted with NaOH), a temperature of 35 and a pressure of 2 atmospheres. Exposure time was 15 seconds. The sheets then were rinsed with water and post-passivated with a very dilute solution of 0.06% chromium-VI-ions and 0.02 chromium- HI-ions, for 15 seconds at 65, followed by drying.

These sheets had a uniform, thin, continuous, deformable gray to yellowish protective layer.

After application of a PVC organosol, these sheets were subjected to a cup drawing test, and the cups were exposed to a dew point corrosion test according to DIN 50017 for 504 hours. The samples withstood the tests faultlessly whereas sheets which had been treated according to conventional chromatization procedures exhibited edge peeling after 504 hours.

(Note: The term DIN denotes Deutsche Industrienormen or, translated, German Industrial Standards, which are comparable to those set in the United States by ASTM.)

EXAMPLE 4 Pot-galvanized sheets which had been degreased were sprayed for 30-60 seconds at 50 with a solution of 0.2 g./l. ironHI-ions, 0.65 g./l. nitrate ions and 0.7 g./l. fluoride ions, having a pH of 1.2. The sheets emerged with a uniform, thin, gray to slightly yellowing protective layer which imparted to them good protection against corrosion.

EXAMPLE 5 A solution for the treatment of galvanized surfaces was produced by dissolving 18 g. iron-III-nitrate-9 H 0 and 24.2 g. sodium hydrogen fluoride in one liter water and adding enough nitric acid to adjust the pH to 1.3.

Pot-galvanized sheets which had been degreased with a neutral cleanser were immersed in that solution for 10- seconds. After a water rinse, a passivating solution of 0.01% phosphoric acid and 0.01% chromic acid was applied, and the sheets then dried. The sheets thus treated had a uniform, thin, deformable protective layer which provided good adhesion for lacquers, paints and synthetic coatings.

The bath could be replenished with a mixture of the starting materials and adjustment of the pH to the original value with HNO and was not in the least impaired thereby.

EXAMPLE 6 In lieu of the solution given in the preceding example, one of the following composition was produced:

5 g./l. Fe-III-ions g./l. Fuoride ions g./l. Nitrate ions.

The pH was adjusted to 1.8 with NaOH.

Hot galvanized sheets were immersed therein for 15- 20 seconds at room temperature and attained a uniform gray to yellowing protective layer which well promoted adhesion to organic coatings subsequently applied.

EXAMPLE 7 Hot dip galvanized steel sheets were treated with a 2.5% solution of a concentrate of the following composition:

18% iron-III-nitrate-9 H O 13.5% HF balance water.

Applied at room temperature for 3 seconds, this solution deposited a uniform, thin, well deformable protective layer.

The bath repeatedly was replenished by addition of proportionate quantities of the concentrate and continued to produce good layers even at high bath load.

EXAMPLE 8 A solution for zinc-clad surfaces was prepared by dissolving.

3.55 g./l. iron-III-sulfate 4.75 g./l. sodium fluoride 8.0 g./l. HNO (62%).

Hot dip galvanized iron sheets were dipped in this solution at 80 for 10 seconds and attained a uniform gray protective layer to which subsequently applied organic 6 EXAMPLE 9 A solution was produced having the following composition:

3.5 g./l. K TiF 7.0 g./l. HNO (62% The pH was adjusted to 2.0 With NaOH.

Pot-galvanized sheets were immersed therein at 25 for 15 seconds. In lieu of the titantiumhexafiuoride complex, corresponding amounts of the zirconiumor silicon hexafluoride complexes were equally effective.

The sheets acquired a uniform gray to yellowish layer which layers were somewhat softer than those produced with ironhexafiuoride-containing solutions.

EXAMPLE 10 Hot dip galvanized steel sheets were treated in a solution containing:

6 g.l. FeCl -6H O 2.9 g./l. HF 70% 10 g./l. sodium chlorate.

The treatment temperature was 65", dwelling time 60 seconds. A uniform, thin and well deformable protective layer formed.

EXAMPLE 11 In lieu of the solution according to Example 10, a bath containing the following components was produced:

8 F2(SO4)3' 4 g./l. HF (70%) 20 g./l. sodium-m-nitrobenzenesulfonate.

The zinc surfaces were immersed at 70 for 30 seconds and attained a uniform, gray to yellowish, thin protective layer. When the bath temperature was lowered to 40-50, the time required was 6080 seconds.

Whereas these examples, when naming pH values, state only those of 1.2-2.0, it has been established that the solutions having a pH of as low as 0.8 and as high as 3.5 are equally well applicable. Merely the time values change somewhat thereby, but it is to be considered that the times also are dependent upon the operational temperatures employed. Times as short as 3 seconds are readily attainable with a pH of 0.8, whereas the treatment time may have to be lengthened to 120 seconds at a pH of 3.5, depending on conditions and intended end uses. The times also vary, of course, with the thickness of the coating layer, as desired and predetermined.

In lieu of the oxidizers specifically named in the examples, all other oxidizers previously mentioned in the body of the specification are equally applicable with substantially like results.

The hot dip galvanized steel or iron referred to is a steel which had been galvanized according to a special pot galvanizing process which entails a rolling treatment of the galvanized metal. The process is known in Germany as Sendzimierverzinken.

I claim as my invention:

1. A process for the application of protective layers of predetermined thickness on zinc and zinc-clad surfaces which comprises contacting said surfaces with aqueous solutions, which are free of oxalic, chromic and phosphoric acids, and which contain complex fluorides of iron and at least one oxidizer soluble in said solution selected from the group consisting of hydrogen peroxide, nitric acid, nitrates, nitrites, chlorates, bromates, nitroguanidine, picric acid, nitrophenols, monoand dinitrobenzenesulfonic acid; the solution having a pH from 0.8 to 3.5, at temperatures ranging from substatnially 5 to C., until the desired layer has formed.

2. The process as defined in claim 1, wherein said solution contains 0.1 to 10 g./l. iron-III-ions, 0.2 to 40 g./l. fluoride ions and 0.3 to 50 g./l. nitrate ions.

3. The process as defined in claim 1, wherein said layer is formed within a time ranging from 3 to 120 seconds.

4. The process as defined in claim 1, wherein the temperature is room temperature.

5. The process as defined in claim 1, wherein the surfaces thus provided with the protective layers are contacted with a dilute aqueous solution of 0.01 to 5 g./l. of an acid selected from the group consisting of chromic acid, phosphoric acid and mixtures thereof.

6. The process as defined in claim 5, wherein the acid treatment is preceded by a water rinse.

7. The process as defined in claim 5, wherein the acid treatment directly follows the original process in the absence of any intermediate steps.

8. The process as defined in claim 5, wherein the acid treatment is carried out with an aqueous chromic acid solution having 0.2-2 g./l. CrO and 005-1 g./l. Cr O 9. The process as defined in claim 1, wherein said complex fluorides of iron are formed in situ by soluble iron salts and a corresponding amount of hydrofluoric acid.

10. The process as defined in claim 1, wherein said solutions are replenished by adding thereto concentrates containing like ingredients as the original solution and having a ratio of free acid to total acid of 1:1.05 to 1.75.

11. The process as defined in claim 1, wherein, subsequent to said contacting step, said surfaces are water rinsed and dried.

12. The process as defined in claim 1, wherein said contacting step is performed by spraying said surface with said aqueous solutions.

13. The process, as defined in claim 1, wherein said contacting step is performed by immersing said surface in said aqueous solutions.

References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-62 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION 3,539 ,403 November 10, 1970 Patent No. Dated Inventor(s) Christian Ries It is certified that error appears in the above-identified patent and that said Letters Petent are hereby corrected as shown below:

Column 5, line 75 after "organic" i nsert coatings adhered well.

Signed and sealed this 6th day of April 1971 (SEAL) Attestz EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, J! Attesting Officer Commissioner of Patents 

